In the smelting and refining of metals, the occurrence of a lower density layer of slag atop the metal surface is common. In many cases, this floating slag layer is purposely provided as a sink for impurities which might otherwise remain in the refined metal, and to prevent oxidation of molten metal in the presence of atmospheric gases. However, as important as the layer of slag may be for achieving its intended purposes, entrainment of slag in metal poured from the furnace or ladle results in a product which must be downgraded, reworked or scrapped.
In the basic oxygen process, for example, the furnace charge consists of scrap steel of varying amount, generally from 15-30% by weight, but up to 45% by weight with preheating, onto which a layer of molten pig iron is poured. Ferrosilicon and other ingredients are added and oxygen injected through a lance. The combination of oxygen with iron, silicon, and other ingredients forms a slag which rises to and covers the surface. The slag comprises nominally about 13 weight percent of the furnace contents, or about 28% by volume. The slag not only serves to retain impurities and further prevent unwanted oxidation, but also serves to keep oxygen and other gases in solution, to avoid effervescence from the melt.
The geometry of basic oxygen and other furnaces varies somewhat, but generally consist of a cylinder with a concave or flat bottom, together termed the "barrel", surmounted by a "cone" whose diameter tapers toward the upper end of the furnace mouth.
In pouring steel from the furnace, past attempts to avoid slag entrainment have included tilting the furnace on its pivot or trunion and decanting the lighter slag from the steel. This method has not proven successful, however, as the hot slag and molten metal were found to adversely affect the refractory lining along the mouth of the furnace. Moreover, it is difficult to remove the slag completely without some molten steel pouring over and coating the rim. Thus, steel is now almost universally withdrawn by gravity flow through a taphole, located generally at the intersection of the cone and barrel of the furnace.
Pouring the steel through the taphole thus described has the advantages of avoiding damage to the rim of the furnace mouth and the risk of forming a layer of steel thereupon. It has the further advantage that the protective slag layer remains floating on the surface of the liquid steel, shielding it from the atmosphere as well as avoiding effervescence. However, as the level of steel in the furnace diminishes, a vortex is created which draws slag into the metal being poured. To avoid this, numerous preventative methods have been devised.
In U.S. Pat. No. 4,431,169, an elongated stopper mounted on a boom is inserted proximate the taphole, and lowered to block the pour when the amount of metal remaining is low. The boom is then raised a small amount, allowing a slow pour of metal from the furnace without creating a vortex. This method utilizes relatively expensive control apparatus and is subject to a great deal of error, because the mouth of the taphole cannot be seen. The error is compounded when the mouth of the taphole has been eroded from use. Moreover, a slight error in the timing of the retraction of the plug from the taphole can allow slag to be entrained in the steel being poured, or worse, could cause blockage of the taphole by steel which has cooled too much due to the slower pour speed with the taphole mouth partially blocked.
In U.S. Pat. No. 4,799,650, a "dart" closure having a higher specific gravity than slag but lower than steel has an elongated hexahedral extension which acts as a vortex inhibitor. When the level of steel decreases to an amount determined by the geometry and density of the device, the elongated extension of the device enters and obstructs the taphole, preventing further pour of steel and slag. Such devices are of lesser usefulness in conventional side-tapped furnaces where the depth of metal above the taphole is limited, thus permitting the device to descend sideways such that the extension passes by the taphole and thus cannot obstruct the taphole at the appropriate time. Moreover, not only is a substantial amount of steel retained in the furnace when the dart enters the taphole, but also the dart is difficult to remove from the taphole. A device having a tetrahedral shape but without the elongated extension is a distinct improvement, as taught by U.S. Pat. No. 5,044,610. This device restricts vortex formation, and allows an increased pour of metal before the device obstructs the taphole. However, even with the '610 device, some slag may yet be entrained in the steel, especially if the operator-controlled furnace tilt angle is far from optimum.
In U.S. Pat. No. 5,203,909, as the amount of steel diminishes, a lance providing a pressurized jet of air or inert gas is positioned above the surface of the metal/slag interface, thereby literally blowing the slag away from the taphole. Correct positioning of the lance is necessary, however, and the use of large quantities of inert gas such as argon increases cost.
In U.S. Pat. No. 4,718,644, a slag sensor is disclosed for mounting on a non-ferromagnetic taphole nozzle. The sensor comprises electromagnetic coils located on opposing sides of the nozzle, and detect the presence of slag by measuring eddy currents and magnetic fields in the material flowing through the nozzle. Unfortunately, such devices do not alert the operator at the time when slag first is entrained in the molten metal, as during this transitional period when both slag and metal exit the nozzle, the relatively large amount of metal is enough to support large eddy currents and magnetic fields. By the time the proportion of slag increases to such an extent that slag is detected, a significant amount of slag has already passed through the taphole and into the ladle. Moreover, the electric motors and reduction gearing which provide the driving force for tilting the large and heavy furnace is only responsive on the order of several degrees of tilt per second. Even if the slag sensor could alert the operator to the onset of slag entrainment, the inertia of the furnace would yet allow for slag entrainment before the furnace is tilted back to a position where the taphole is above the slag.
Despite the many attempts to maximize metal yield while minimizing slag entrainment, the predominant technology in use today is a combination of vortex-reducing floats having a specific gravity between that of slag and that of steel, and operator control of the tilt of the furnace to regulate flow of steel through the side-mounted taphole. As both the slag and steel are intensely hot, emitting enormous amounts of both visible and infrared radiation, the operator cannot easily determine visually the level of steel hidden below the slag layer.
Furnace linings, in general, are quite thick, for example in excess of two feet thick with an additional "safety" lining of from 6-9 inches. Such linings are replaced after from 5000 to 6000 heats. During the initial campaigns, the thickness of such linings and attendant volume of the furnace prevent the furnace from being tilted past 97-98.degree. during the last portion of a pour. As the furnace matures, the lining is eroded and the pour angle increases until it reaches a value of from 110-111.degree..
Determination of the slag content of the furnace is thus not only difficult, but moreover, this determination is rendered more difficult by the natural wear and erosion of the furnace refractory interior. In addition, the error is compounded by the normal variance in observation and reaction, particularly with respect to differences in operator experience and skill level, attentiveness, and the like. Thus, the industry still awaits a satisfactory solution to slag entrainment and yield maximization.